Combustion engines for passenger cars have to fulfil a large number of requirements. These include emissions and noise legislation, good fuel economy as well as drivability and comfort issues. Future emissions legislation will further tighten the limits of the emissions of nitrous oxides, NOx, hydrocarbon compounds, HC, carbon monoxide, CO and particulate matter, PM, measured in steady state as well as dynamic tests.
As a result, it will be necessary to provide engine components, e.g. valves for exhaust gas recirculation (EGR), turbo chargers, injectors and nozzles, as well as the sensors, the computing capacity and the appropriate algorithms to precisely meter air, recirculated exhaust gas (EGR) and fuel, not only at steady state but also at transient conditions of engine load and speed.
One of the key technologies for the development of novel combustion processes are advanced injection strategies. Precise and repeatable metering of smallest quantities of fuel as well as excellent atomisation at different engine operating conditions are only some requirements for these strategies.
Up to five injections per cycle, with hydraulic dwell times between zero and some 30 degrees crank angle, are supposed to reduce pollutant emissions and at the same time yield acceptable fuel consumption. It is believed that in order to obtain a strong reduction of smoke as well as NOx emission in part load, 4 injections instead of 2 should be applied.
Single injections with an optimised rate shape might be an alternative. In part load, NOx emissions were found to be reduced best for boot and least for square shaped injections, assuming a constant crank angle position of 50% heat release (HR50). Smoke emissions were minimal at ramp and increase towards square and boot. At low engine speeds and high loads only minor benefits could be obtained for the boot shaped injection.
For piezo driven common rail systems, the boot shaped injection can possibly be achieved by using several consecutive injections with very small or even negative hydraulic dwells. These dwells can be realised, since the piezo actuator can be energised immediately after the de-energising of the previous injection. This enables the re-opening of the nozzle needle at the end or even during the closing phase of the preceding injection. This however includes the risk, that atomisation might deteriorate, due to the multiple opening sequences of the nozzle needle.